1. Field of the Invention
This invention relates to motor starting contactors with an overload relay system and, more specifically, the invention relates to an apparatus and method for displaying certain conditions of the motor controller and the associated motor.
2. Background Information and Description of the Prior Art
Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is often called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil which is controllable upon command interacts with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. The armature has a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged by the movable contacts as the magnetic circuit is engaged and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts.
When the system is used as a motor controller with an overload relay device, the overload relay function senses current which is related to the input current being drawn by the motor. This then is provided to an electromagnetic or electrothermal device calibrated to cause the main contacts to open under certain overload current situations. Additionally, raw data concerning the current is supplied to a microprocessor. Typically, the microprocessor calculates the heat generated in the motor by calculating current squared multiplied by time, or I.sup.2 t. It is necessary when supplying the current to a microprocessor that it be converted from an analog to a digital signal for effective utilization by the microprocessor. An apparatus for use with an electromagnetic contactor and the background information concerning an exemplary electromagnetic contactor is provided in U.S. Pat. No. 4,757,420 issued Jul. 12, 1988 and which is assigned to the assignee of the present invention.
As discussed in the U.S. Pat. 4,757,420 the current is measured and a representative signal is generated for input to the microprocessor. As mentioned above, prior to supplying this signal to the microprocessor it is necessary to convert the analog representation thereof to a digital signal for effective utilization by the microprocessor. The microprocessor thereafter performs various calculations in order to determine from the current representative input signal, the heat in the associated motor. This value is then used to communicate with the motor controller to open the contacts of the motor controller in overload current situations or for other purposes within the system. An operator of the system, however, does not have a convenient way of determining the status of the system at any particular point. For example, the operator does not know whether the system is near a trip condition because the operator has no direct information about the thermal condition of the motor. Thus, the only indication of the thermal condition that the operator has is when the motor controller trips in response to a current overload situation. If the operator were aware of the current status of the thermal condition of the motor, the operator could take steps to avoid an early trip of the motor controller. In other words, an operator who is activating a motor starter in order to effectuate a particular task could slow the activations down or space them apart at greater time intervals in order to allow a further cooling of the motor between activations. At present, there is no method by which an operator can visually check the system to determine the status. There remains a need, therefore, for a device which would allow an operator to visually check the status of the motor controller and the associated motor in order to control activation of the system to more efficiently operate the motor and the motor starter.
In addition, after a motor has tripped, typically a safety feature is built into the motor controller such that it cannot be reset until the motor cools down to a certain percentage of the trip value. The operator, however, is not aware of the degree of cooling within the motor. This means that the operator would simply continue to press a reset button until the device resets. There remains a need for a device which indicates whether a reset is allowed which would thereby indicate that the motor has cooled to a sufficient temperature to allow operation thereof after a trip condition has occurred. There remains a further need for a device which indicates that a trip has in fact occurred and a device which indicates that the system is ready to be reset and thereby reactivated.